I have not seen this mentioned anywhere, but the more heavily thoriated a filament is, the more brittle it becomes. In order to achieve enough emissivity without increasing the filament power, I speculate that the filament of the Elrog is very heavily thoriated. (For example, the 71A has a 0.25-amp oxide filament, while the original 71 had a 1.0 amp thoriated tungsten filament.)

If my speculation is correct, then shipping the tubes would be more risky than other tubes.